1. Field of the Invention
The present invention relates generally to a recording disk driving system in an information recording apparatus rotating a recording medium in a disk form at a constant speed and accessing the recording disk for information and, more particularly, to a recording disk driving system in an information recording apparatus using a stepping motor as the drive source for directly driving a recording medium.
2. Description of the Prior Art
For writing information into and reading information from a disk-formed information recording medium used in a word processor or a personal computer, called a floppy disk, a floppy disk drive (FDD) is used. In the FDD, a brushless DC motor has so far been used as the spindle motor for rotatably driving the floppy disk. Although there are no problems with the brushless DC motor as to its characteristics and reliability, it requires position detectors and a speed detector for its rotor. Therefore, the cost of the motor and driving device becomes high. More specifically, in order to ensure accuracy in the rotation, a servo control with speed feedback technique becomes necessary in the driving and this causes the circuit configuration to become larger in scale. Thus, it has become impossible for the system to meet the recent requirements for cost reduction.
Under these circumstances, studies have so far been made on the stepping motor as to its use for the spindle motor for driving the recording medium. Since the stepping motor operates stepwise by the nature of its configuration, in order to rotate it smoothly while keeping accuracy in rotation of the recording disk, such a method has been used as to increase the moment of inertial of the rotating portion and improve the rotational operation by means of the flywheel effect. However, as a natural consequence of the increase in the moment of inertia, it has become difficult to cause the recording disk to self-start and hence provision of a dedicated circuit for starting the recording disk has come to be needed.
The starting circuit for the stepping motor is generally called a ramp circuit or throw-up circuit (hereinafter called a ramp circuit) and it functions so as to excite the motor with a very low frequency at the beginning of the starting and then increase the exciting frequency gradually until it becomes synchronized with the rated revolution. In the case of a stepping motor provided with no rotor position detector, a starting system best suited to the load characteristic of the apparatus becomes necessary to ensure reliable starting and obtain a short revolution settling time.
The term "revolution settling time" in the present invention means the time period from when the recording disk in its stopped state is started up to when the recording disk then accelerated acquires such an average number of revolutions, accompanied by accuracy in the rotation, that allows access to information. When referring not to the time itself required for settling the revolution but the general rise in the number of revolutions, the term "starting characteristic" is used.
An information recording apparatus, especially a floppy disk drive has an extremely great frictional (load) torque because the magnetic head is always in contact with the recording medium and the value of the torque greatly varies depending on the track position on which the head is placed and the environmental conditions of the contact portion. In a 3.5 -inch FDD, for example, the value varies from 1.0 to 10.0 (mN.multidot.m).
In the worst case, it sometimes occurs that the head sticks to the medium.
In the case of a stepping motor provided with no rotor position detector, even if equal starting current is supplied, the generated torque greatly varies depending on the magnetic relative position between the rotor and the stator at the time of start-up. This not only causes fluctuation in the revolution settling time (unstable state) but also causes, when the torque margin is not sufficient, disk trouble, even if a ramp circuit is used, being still unable to self-start (failure of start-up).
Therefore, when a stepping motor is used for driving the recording disk in an information recording apparatus, it becomes necessary to provide a driving system ensuring stable starting and having high reliability even under the conditions of a widely varying load and an extremely great moment of inertia of the recording disk.